They presumably feel that without the apostrophe, it would look like it’s pronounced “ban-joss”. As if people in front of a musical instrument store would no longer recognise the word “banjo” after it’d had some newfangled pluralization-type thing stuck on the end of it.

It’s really simple, Joan: “…guitar ends with a consonant, banjo with a vowel. You must put an apostrophe at the end of any word to make a plural if it ends with a vowel…”
…or so I was told by a former boss. Stress on ‘former’!

OK, so what makes one think guitars should be plural, but banjo’s should be possessive? OK, so they probably don’t get that at all, so we’ll take it down to their level – what makes one think one should be “s” and one should be “apostrophe s”?????